The role of the metal formal oxidation state in metallaborane structures relating to closo/isocloso and nido/isonido pairs

Relationships have been observed between the metal formal oxidation state and the metal valence electron counts in closo/isocloso and nido/isonido pairs of polyhedral metallaboranes. In general, the metal formal oxidation state increases by two units in a closo → isocloso or nido → isonido structural transformation, generally involving diamond–square–diamond processes relating to internal metal oxidative addition reactions. More exotic “isoiso” structures are found for metallaboranes containing metals exhibiting relatively high oxidation states such as molybdenum, tungsten, and rhenium. The free borane ligand obtained by removal of the metal vertex from a metallaborane structure can be classified as symmetric or asymmetric depending upon the nature of the metal–ligand bond. In symmetric borane ligands found in closo metallaboranes, the metallaborane structure is generated by symmetrical “centroid” metal capping of the ligand open face. However, in asymmetric borane ligands found in nido metallaboranes, the metallaborane structure is generated by skew-symmetrical or offset metal capping of the ligand open face. Capping regime descriptors are defined to describe these types of metal–borane bonding. In addition, electrostructural correlation tables are described for closo/nido and nido/arachno pairs relating metal formal oxidation states, the numbers of metal valence electrons, and the capping regime descriptors.

The metal formal oxidation state increases by two units in a closo → isocloso or nido → isonido structural transformation, generally involving diamond–square–diamond processes relating to internal metal oxidative addition reactions. More exotic “isoiso” structures are found for metallaboranes containing metals exhibiting relatively high oxidation states such as molybdenum, tungsten, and rhenium.Figure optionsDownload as PowerPoint slide